The present disclosure relates generally to the electrical, electronic and computer arts and, more particularly, to contact metallization processes and contact structures.
With shrinking dimensions of various integrated circuit components, transistors such as FETs have experienced dramatic improvements in both performance and power consumption. These improvements may be largely attributed to the reduction in dimensions of components used therein, which in general translate into reduced capacitance, resistance, and increased through-put current from the transistors. Metal oxide semiconductor field-effect transistors (MOSFETs) are well suited for use in high-density integrated circuits. As the size of MOSFETs and other devices decreases, the dimensions of source/drain regions, channel regions, and gate electrodes of the devices, also decrease.
The contribution of middle-of-line (MOL) contact resistance to the total parasitic resistance is increasing due to aggressive dimensional scaling in advanced CMOS devices. Titanium (Ti), titanium nitride (TiN) and tungsten (W) layers are used as liner/barrier/nucleation layers in MOL metallization and help ensure adequate adhesion. These layers occupy an increasing amount of the interconnect volume. Interfacial contact resistance between two interconnect levels is dominated by liner/barrier/nucleation layers of some devices.
Silicon-based devices typically include multiple interconnect metallization layers above a device layer that contains field-effect transistors (FETs), memory devices, or other structures. The metallization layers often include high aspect ratio holes or vias that are relatively deep and have narrow diameters. The ratio of the depth to the diameter (aspect ratio) of contact holes is often much greater than one. Tungsten (W) is frequently employed as a fill material for contact holes having narrow (sub-micron) diameters. Tungsten fill material is deposited conformally using, for example, low pressure chemical vapor deposition (LPCVD). As the dimensions of tungsten metal contacts are scaled down for future technology nodes, currently used metallization techniques may not be able to deliver structures that meet the resistance targets required. The need for thick TiN liners to prevent fluorine diffusion and poor metal gap fill due to seams or other void types contribute to the high resistance of scaled-down tungsten contacts. Such liners reduce contact hole diameter and thereby displace metal conductor volume that could otherwise be deposited within the contact hole.
Currently employed conformal deposition processes for damascene tungsten involve the use of tungsten hexafluoride (WF6) as a source material for the tungsten. The TiN liner protects the underlying titanium and silicon from the fluorine that is released during deposition. The process of filling high aspect ratio contact holes using conformal deposition processes often results in the formation of vertical seams within the metal contacts. Such seams are formed near the convergence of the tungsten-coated side walls of the contact holes. The seams also displace metal conductor volume and could lead to higher resistance.